1. Field of the Invention
The present invention relates generally to an apparatus and method for beamforming in a multi-antenna system, and in particular, to an apparatus and method for performing beamforming depending on feedback information.
2. Description of the Related Art
Mobile communication systems are evolving into high-speed, high-quality wireless packet data communication systems capable of providing data service and multimedia service beyond the early voice-oriented service. The ongoing standardization on High Speed Downlink Packet Access (HSDPA) being led by 3rd Generation Partnership Project (3GPP) and 1× Evolution Data and Voice (EV-DV) being led by 3rd Generation Partnership Project-2 (3GPP2) are efforts to find a solution for a high-speed, high-quality wireless packet data transmission service at 2 Mbps or higher in a 3rd generation (3G) mobile communication system. A 4th generation (4G) mobile communication system aims at providing a high-speed, high-quality multimedia service at a rate much higher than that of the 3G mobile communication system.
Multipath interference, shadowing, propagation attenuation, time-varying noise, and interference in a wireless channel environment are factors deteriorating the high-speed, high-quality data service. Therefore, much research has been conducted on techniques for overcoming these factors. Error control coding technique and a multi-user diversity scheduling are typical techniques used to improve the quality of service.
The multi-user diversity scheduling technique detects channel conditions of mobile stations requiring packets, based on feedback information, and preferentially transmits a packet to the mobile station having the best channel condition, thereby acquiring a diversity effect including a signal-to-noise ratio gain. A diversity order indicating a level of the diversity gain corresponds to the number of mobile stations that simultaneously require packets.
The multi-antenna technique uses a spatial domain as an additional resource to acquire a higher data rate. The multi-antenna technique is implemented based on a multi-antenna system in which a transmitter and a receiver both have multiple antennas, and a beamforming technique.
The beamforming technique arranges a plurality of transmission antennas and separately adjusts weights of signals for each of the transmission antennas, so that the same frequency-domain signals can have different gains according to their directions. The beamforming technique, as it uses a geometric, spatial axis, extends an operating frequency bandwidth through a nulling process. In other words, the beamforming technique can overcome the limitation of frequency resources.
The beamforming technique is popularly used for improving a signal-to-interference plus noise ratio (SINR) by reducing interference signals on the same channel in a wireless communication system operating under the cell plan such as a mobile communication network or a wireless local loop (WLL).
The beamforming technique is implemented on the assumption that channel conditions can be correctly detected. Therefore, for the beamforming technique, there is a need for feedback of information based on which a transmission side can estimate channel conditions or a reception side can detect channel conditions. Preferably, the beamforming technique detects channel conditions based on feedback information from the reception side. The feedback information, which is channel information, represents channel responses between the transmission antennas and the reception antennas. The channel information increases with the number of transmission/reception antennas.
Therefore, if it is desired to detect correct channel conditions or the number of transmission/reception antennas increases, the required amount of the feedback information increases. The increase in the amount of the feedback information increases the amount of radio resources used for transmitting the feedback information, causing an increase in radio traffic. In addition, the increase in the amount of the feedback information increases the number of calculations for beamforming, and the increase in the number of the calculations increases a load on the system.
For these reasons, there is a demand for a plan to minimize the amount of required feedback information, without affecting performance that can be obtained by the beamforming technique.